the omicron subvariant ba.2 has now become the dominant strain of covid in the u.s. accounting for more than 50% of all cases. that's according to the cdc. it comes as the fda expands the emergency use authorization of the pfizer and moderna vaccines, which allows adults 50 and older to get a second booster shot, which means four shots overall. those eligible are able to get that shot four months after their first booster shot. council to the u.s. says everyone should take advantage of. >> the boosters work, and americans should get their booster. there is waning across time. so it's important people take advantage of the extra protection. >> in shanghai, nearly 6,000 new covid infections, accounting for